The present invention relates to a process for producing a positive electrode active material for alkaline storage batteries.
With the progress of semiconductor technologies in recent years, commercialization of electronic equipment with reduced size, lighter weight and more multi-functions is advancing, and the personal use of small-sized portable equipment, typically represented by portable telephones and notebook-sized personal computers is progressing rapidly. The requirement for alkaline storage batteries, which are in wide use as the power source of such equipment, with smaller size and lighter weight is also becoming increasingly strong.
To date, the main active material used for the positive electrode of alkaline storage batteries is nickel oxide (NiOOH); as to the electrode substrate, an electrode comprising a three dimensional foamed nickel porous body with higher porosity (95%) and nickel oxide powder filled in high density thereinto (foamed metal type electrode) has been industrialized (for example, JP-B-62-54235 and U.S. Pat. No. 4,251,603) to replace the sintered-type electrode which uses the previous sintered substrate, and resultantly the energy density of nickel positive electrodes has greatly improved.
In attaining the high energy density of nickel positive electrode mentioned above, improvement in the process for producing nickel oxide powder of the active material was also one of the important technologies. The process for producing nickel oxide powder previously used comprises reacting an aqueous solution of alkali, such as sodium hydroxide, on an aqueous nickel salt solution to form a precipitate, then aging the precipitate to effect crystal growth, and pulverizing the large granules thus formed by mechanical means. This process has the disadvantages of complicated procedures and low filling density resulting from the irregular shape of powder particles. However, another process has been proposed, as shown in JP-B-4-80513, which comprises reacting ammonia on an aqueous nickel salt solution to form an ammonium complex of nickel and then making nickel hydroxide grow in an aqueous alkali solution; resultantly, a continuous production process has become adoptable to reduce the production cost, and additionally high density filling has become possible because the shape of particles thus obtained is close to spherical.
However, the paste-type electrode of this kind, which uses high density particles of large particle diameters grown to several ten .mu.m as the active material, has the difficulty of decreased charge-discharge efficiency due to the decreased electric conductivity of the active material itself. To overcome the difficulty, the improvement of charge-discharge efficiency has been attempted by adding Co, its oxide, Ni, etc. to supplement the electric conductivity (JP-B-61-37733; Denki Kagaku (Electrochemistry), Vol. 54, No. 2, p. 159 (1986); Power Sources, 12 p. 203 (1988)) and further, with the intention of improving also the active material itself, by incorporating thereinto metal elements other than Ni, such as Co, as solid solution. The latter attempt to improve the charge-discharge efficiency by incorporating a different kind metal element to the inside of crystals as solid solution has hitherto used, as shown for example in JP-B-3-26903, JP-B-3-50384; Denki Kagaku (Electro-chemistry), Vol. 54, No. 2, p. 164 (1986); and Power Sources, 12, p. 203 (1988), the method of adding Cd and Co to the inside of the active material. For reasons of environmental issues, however, cadmium-free batteries are eagerly desired and Zn has been proposed as an example of metal elements for replacing cadmium and, further, solid solutions of such 3 elements as Co, Zn and Ba have been also proposed (U.S. Pat. No. 5,366,831). Such addition of different kinds of metals to nickel oxide to form a solid solution, intended for improving charge-discharge efficiency, is a long-standing technique and was already disclosed in, for example, JP-A-51-122737.
The improvement in the structure of substrates, the shape of active materials, the composition of active materials and the additives described above has greatly improved the energy density of positive electrodes, and at present positive electrodes with an energy density of about 600 mAh/cc have become practical. However, continual need for improving the energy density of batteries as the power source of small-sized portable equipment is increasingly strong. The improvement of energy density of batteries may conceivably be approached from such aspects as the positive and negative electrodes, electrolyte, separator and method of construction of these. With regard to the method of constructing a battery, a rapid increase in energy density has been attained by such technological progresses as the use of thin separators and high density filling of electrode plates, and the increase has nearly reached its limit at present. As to the negative electrode, the practical use of metal hydrides of high energy density (Power Sources, 12, p. 393 (1988)) in place of the previous cadmium negative electrode have brought a volume energy density of negative electrode two or more times that of the positive electrode. Therefore, to attain a still greater increase in energy density of a battery, the improvement of the positive electrode is effective; that is, a still greater improvement in the energy density of the positive electrode, which occupies nearly the half the volume of a battery, has become indispensable.
Improvement in the energy density of the positive electrode may conceivably be approached by improvement in the electrode filling density, for example, improvement in the tap density of active materials, decrease in the amount of additives and decrease in the amount of metal of the foamed nickel substrate, but these techniques are approaching nearly to their limits. Therefore, it is necessary to aim at modifying the active material itself, improving the reactivity and improving the order of reaction. It is generally considered that the nickel oxide of the positive electrode active material used at present is in the form of .beta.-type Ni(OH).sub.2 (di-valent oxide) at the time of filling and, in usual charge and discharge, a one-electron reaction (utilization: 100%) proceeds between it and the .beta.-type NiOOH (tri-valent). However, the .beta.-NiOOH at the charged state is oxidized by overcharge partly to a higher order oxide, .gamma.-NiOOH (3.5-3.8-valent). Heretofore, various methods have been devised to suppress the formation of the .gamma.-NiOOH because it not only causes voltage drop and capacity decrease owing to its electrochemical inactiveness but causes various additional troubles as imperfect contact of the electrode with the conducting agent and the substrate due to the volume expansion of the electrode caused by spreading of .gamma.-NiOOH into the space between the layers formed by Ni--O, falling of the active material, and exhaustion of the electrolyte caused by the .gamma.-NiOOH incorporating water molecules thereinto. However, in order that a high energy density may be attained by using an active material based on nickel oxide, it is essential to use the .gamma.-NiOOH of the higher order oxide. Accordingly, materials of a structure similar to .alpha.-type hydroxide in which a part of Ni is replaced with a different kind metal, e.g., Mn(III), Al(III) and Fe(III), to form a solid solution, and anions and water molecules are incorporated between the layers have been proposed (for example, Solid State Ionics, 32/33, p. 104 (1989); J. Power Sources, 35, p. 249 (1991); U.S. Pat. No. 5,348,822 (1994), U.S. Pat. No. 5,569,562 (1996), and JP-A-8-225328). It is described that charge and discharge reactions readily proceed between this oxide and the higher order oxide of a structure similar to .gamma.-NiOOH. Actually, however, the oxide has a wide interlayer space even at the discharged state and hence the material itself is very bulky, so that the oxide is conceivably difficult to fill in high density and difficult to use in practice. To solve these problems the present inventors have paid attention to an active material which has a .beta.-type crystal structure at the time of electrode filling and which can make a charge-discharge reaction proceed between itself and the .gamma.-NiOOH of the higher order oxide.
The present inventors have proposed various complex metal oxides which are sometimes referred to as multiple metal oxides. The complex metal oxides comprise nickel oxide containing a metal element other than nickel added to the inside of the crystals of the nickel oxide, but the complex metal oxides have various problems, including the lowering of charge efficiency and of charge-discharge voltage at high temperatures due to the lowering of oxygen overvoltage, and the lowering of chemical or electrochemical stability, depending on the kind of metals added and the composition of the oxides. Accordingly, the present inventors, from the necessity of improving the property of the interface between the active material and the electrolyte, are directing attention to an active material having a complex metal oxide layer coated thereon which has characteristic properties of 1 a high charge efficiency at high temperatures, 2 a high discharge voltage and 3 high chemical and electrochemical stabilities.
A process for producing a surface coating layer hitherto proposed is a batch type one which comprises first synthesizing the oxide of the inner layer, followed by water-washing and drying, and then growing the oxide of the surface layer in a separate deposition vessel. With regard to a process for producing nickel hydroxide coated with cobalt hydroxide, though it is intended for a material different from the above-mentioned oxide comprising mainly nickel, a process has been proposed wherein the coated nickel hydroxide is produced continuously by connecting a vessel for growing nickel hydroxide of the inner layer, a vessel for water-washing and a vessel for coating cobalt hydroxide, successively (Japanese Patent Application No. 7-40853). In this application, however, since it aims at coating cobalt hydroxide, a water-washing vessel is provided between the nickel hydroxide deposition vessel of the primary stage and the cobalt hydroxide deposition vessel.
A process hitherto proposed for producing a complex metal oxide containing Co, Zn and Cd added thereto as the metal other than Ni comprises mixing the respective predetermined amounts of a Ni salt and respective metal salts, dissolving the mixture in water to form an aqueous solution, reacting ammonia on the aqueous solution to form an ammonium complex, and then adding an aqueous alkali solution to the resulting reaction mixture to effect neutralization. In this case, since the amount of the different kind metals added is small, approximately the same conditions were adopted as in the process for producing nickel oxide for such conditions as reagent concentration, pH, temperature and residence time.
However, in cases where metals other than Ni in metal oxides are of many kinds or of a large amount, particles of uniform composition and high density were difficult to obtain because, depending on the kind of metal salt some metal salts may form salts of very low solubility when mixed with aqueous solutions of other metal salts or they may deposit as precipitates owing to the change of pH taking place when ammonium ions are reacted on the salts.
Furthermore, since the process for producing complex metal oxides which comprise nickel as the main metal element and have different compositions and kinds of constituents between the surface layer and the inner layer is conducted batch wise, the process has disadvantages of complicated process steps and low mass productivity.
The present invention aims at solving the above-mentioned problems and makes it possible to obtain high density complex metal oxides by properly selecting such conditions as reagent feeding method, reagent concentration, pH, temperature and residence time. According to the present invention, furthermore, in a process for producing complex metal oxides continuously by reacting aqueous solutions of salts of a plurality of metal elements with an aqueous alkali solution through a plurality of successive multi-stage reaction-deposition vessels, by changing the composition and/or the kind of salts of a group of metals, which are to form their oxides, in the reaction-deposition vessels of the stages adjacent to each other, composite oxides which comprise nickel as the main metal element and are different in composition and/or kind can be group in respective reaction-deposition vessels, and hence positive electrode material particles formed of a plurality of layers of the above-mentioned oxides can be produced continuously. Accordingly, the process steps can be simplified and the mass productivity can be improved as compared with previous batch processes.